The present invention relates to a multiple access system for transmitting and/or receiving radio signals.
Many telecommunication applications require message acknowledgment. For instance, the GSM system requires that a data packet formed for transmission is transmitted as several bursts, which are de-interleaved and channel decoded at reception. Then the whole received data packet is searched for errors. If the data packet contains errors, then information about this is sent to the transmitting equipment, which then re-transmits the whole data packet. Message acknowledgment is typically information of the on-off type, in other words, the acknowledgment receiver must only know whether its message was correctly received or not, and act correspondingly. Such situations occur often e.g. in connection with automatic re-transmission (ARQ, Automatic Repeat reQuest), whereby the receiving end informs the transmitting end whether the message was correctly received or not. This is called acknowledgment. If re-transmissions occur seldom, it is generally preferred to send so called negative acknowledgments, or only unsuccessful transmissions are acknowledged, whereby the acknowledgment always represents a request for re-transmission. In this way it is possible to save capacity of the return channel or acknowledgment channel.
When the said ARQ method is used in a multiple access environment, where several users use the same transmission medium, the users' transmissions are distinguished by known methods, e.g. time, frequency or code division techniques. In the same way we try to classify the acknowledgments. If each user sends the acknowledgments rather infrequently, it is does not pay to reserve separate acknowledgment capacity for each, but several users can share a certain "acknowledgment channel". However, then it is possible that more than two users will try to acknowledge at the same time, using the same "acknowledgment channel". Typically all simultaneous acknowledgments are lost when a collision occurs.
An example of the application of re-transmission is presented in the Finnish Patent application FI-924943, which uses fast re-transmission when transmitting speech data or other data which is delay critical, and in which a burst or a part of it is re-transmitted during the time slot used by the speech encoder to generate a new speech frame, whereby the re-transmission does not create any extra delay. A radio telephone system like this requires an acknowledgment channel, in which the receiver can request (the request is automatic) fast re-transmission. In a system where data is transmitted in both directions the acknowledgment can use the normal transmission burst/time slot of the opposite direction. In a system where data is transmitted in one direction only, whereby we talk about a discontinuous transmission (DTX) or utilize e.g. the silent moments in the speech and thus free capacity for other users, we need a separate acknowledgment channel for the re-transmission requests. The re-transmission requests from the base station to the mobiles (the down-link communication direction) may be combined and as several requests transmitted et the same time in one time slot/burst. A problem will be the transmission of acknowledgments from the mobile to the base station (the up-link communication direction). A defined time slot for the acknowledgment transmission can be reserved for each user, but this means wasted capacity, if the re-transmission requests are infrequent. On the other hand, said collisions will occur, if several users are allowed to use the same time slot for the acknowledgments.
As an example, let us discuss the TDMA system, which has 140 time slots in each TDMA frame. Let 100 of these 100 time slots be reserved for normal transmissions, 20 for re-transmissions and 20 for acknowledgments. Thus 100 users can use the same carrier simultaneously. Let as an average 15 to 20% of the bursts request (acknowledge) and re-transmit, whereby on the average 20 time slots for the re-transmissions and 20 time slots for the acknowledgments is sufficient. Acknowledgment from the mobiles to the base station the (up-link direction) is more problematic than the reverse direction, because the mobiles cannot mutually coordinate their transmissions, but several mobiles may try to acknowledge in the same time slot which creates a collision.
The Finnish Patent application FI-925472 presents a communication method and system having as the object to increase the number of users in a network or a cell. Data is transmitted using a combined CDMA/DMA or TDMA/CDMA multiple access, whereby complementary code sets (perfectly orthogonal codes) are used as spreading codes in the CDMA. When the method is applied in operating or planned TDMA systems a CDMA section is included, with which a CDMA multiple access spreading codes as above are used to increase the number of users. The presented method does not require power saving methods or multiple access interference suppression methods to increase the capacity. However, such a communication method and system as such is not suitable to be used in a conventional TDMA system in order to avoid on-off type information collisions, such as collisions of message acknowledgment and channel allocation information, when it is desired to keep the same frequency band, because the CDMA multiple access method broadens the frequency band (spectrum).